Infragravity Waves: Part 1

Infragravity Waves: Part 1

Along the coast where I live, you can
normally find at least one or two rideable spots even in the biggest, stormiest
conditions.That’s what I thought during
the first few years I lived here anyway.Recently, however, I’ve realized that simply heading for the most
out-of-the-way sheltered cove doesn’t always guarantee rideable surf.If the waves are so huge that you end up at
some spot half way up an estuary or behind three or four breakwaters, something
can still make the waves unrideable even if they are down to a reasonable
size.

The problem, in crude terms, is that there
is ‘a lot of water moving’ – something I’m sure you are familiar with.One thing you might not be so familiar with
is just where all that moving water comes from, and why it always seems to be
there at sheltered spots in big storms.

In this, the first part of a three-part
article, I’m going to introduce the concept of infragravity waves – extra long period waves that manage to find
their way up estuaries, around headlands and behind breakwaters, actually
getting bigger while the ordinary waves are getting smaller.

If you are looking for rideable surf during
a huge storm, the further you get from the main exposures, the more you start
to notice large surges of water at the shoreline.Even if the ordinary waves are only a few feet
high, the shoreline itself moves in and out hundreds of metres, quite often
flooding over dunes and car parks and into shops and cafes, and then turning
around and drawing back out again, sucking all sorts of debris with it.The line-up, if there is one, is typically a
wash of swirling currents as all that water moves beneath it, making surfing
uncomfortable and dangerous.

During the last couple of winters, I’ve
noticed a lot of this sort of thing going on along the coast where I live.On the occasions that I’ve gone down to check
the damage the day after the storm, I could see evidence of infragravity waves
everywhere.At the most affected spots
there were whole areas of squashed grassland, with piles of seaweed and other
debris carried a long way inland by the infragravity waves.In some cases they had reached over a
kilometre inland, often well out of sight of the sea.

If you didn’t know what these waves were,
you might think that they were small tsunamis.After all, they do have the same characteristics and behave in pretty
much the same way.However, they come at
regular intervals of five or ten minutes rather than just once, and they are
not associated with any particular geological event, which means that they are
not tsunamis.The fact that they only
appear during large stormy surf suggests that they are probably somehow
associated with the wind-generated waves.

Infragravity waves are a keen subject for
coastal researchers, because they are thought to be an important contributor to
coastal erosion, as well as being dangerous for bathers, coastal road users and
waterfront hotel owners.They were first
documented in the late 1940s and early 1950s by Walter Munk and other legendary
oceanographers.They were originally
referred to as ‘surf beat’ and were acknowledged to have something to do with
the wave groups, or sets.While the
period of the waves we normally see breaking on beaches rarely gets above about
20 seconds, the period of infragravity waves is more akin to the period of the
wave groups, ranging from about 30 to 300 seconds.

Why are they called infragravity
waves?The term ‘infragravity’ is
actually slightly confusing.The prefix
‘infra’ comes from the Latin for ‘below’; and the ‘gravity’ part comes from the
fact that most ocean waves are categorized as ‘gravity waves’ because the restoring force (that which pulls them
down again after the wind has pushed them up) is gravity.Therefore, infra-gravity waves are gravity
waves, but ‘lower’ than normal ones.But
‘lower’ what?Well, wave physicists like
to talk in terms of frequency, the inverse of period.Infragravity waves have a lower frequency
than ordinary waves, which means that they have a longer period.Confused?Well, if you want, you can refer to them as ‘long waves’ which is really
saying the same thing.But infragravity
waves are not just waves that are longer than normal ones; they are a bit more
special than that.

With harbours, estuaries and headlands, the
ordinary waves gradually lose energy as they propagate in and around all the
corners, whereas the infragravity waves just plough on right through to the
most far-reaching pocket beaches and boat ramps.But infragravity waves have mostly been
studied on gently-sloping beaches, where it is easy to compare their behaviour
with that of the ordinary waves.Here,
the ordinary waves dissipate their energy through breaking, while the
infragravity waves just keep on going right to the shoreline.

One important feature of infragravity waves
is that they actually increase in size as they approach the shore.Because of their exceptionally long
wavelength, they never get steep enough to break, not even on the most
gently-sloping beaches.But as they come
into shallow water they do slow down, which causing them to ‘jack up’ in the
same way as ordinary waves do when they hit shallow water.Because the front of the wave slows down
before the back, the wave is squashed up horizontally and pushed up
vertically.In fact, because
infragravity waves never break, they keep on growing like this all the way to
the shoreline and reach their maximum size at the shoreline itself.This process is greatly enhanced during large
stormy conditions (see Figure 1).

In contrast, the ordinary waves, which have
a much shorter wavelength, generally become steep enough to break before they
reach the shoreline.Once they break and
become lines of rolling whitewater, they start to dissipate their energy.On gently-sloping beaches, they tend to break
a long way out, and can lose practically all their energy before they get to
the shore.This is why the lines of
whitewater arriving at the shoreline are usually quite weak and dribbly, even
if the waves breaking out the back are really big.In this case, the ordinary waves are said to
be saturated.In theory, completely saturated waves
diminish to nothing at the shoreline.

Now, if the offshore wave height changes,
the ordinary waves at the shoreline stay the same size (very small), but the
infragravity waves change in response to the offshore wave height.If the offshore wave height gets bigger, the
ordinary waves simply start breaking further out in deeper water, dissipating
that extra energy over a greater distance.No matter how big the offshore wave height gets, the ordinary waves
never get any bigger at the shoreline (Figure 2).In summary, any changes in offshore wave
height are only manifest at the shoreline as changes in the size of the
infragravity wave motions.

As a result, the
shoreline becomes more and more ‘infragravity-dominated’ as the offshore wave
height increases.This concept, which is
very important for coastal oceanography, was proven in the early 1980s in a
series of classic field experiments, which I am going to talk about in Part 2
of this article.

Figure
1: Infragravity waves are so long that they never break, therefore they squash
up and get bigger as they slow down in shallow water, reaching their maximum
height at the shoreline itself

Figure
2: In truly saturated conditions on gently-sloping beaches it doesn’t matter
how big the offshore wave height gets, the ordinary waves still diminish to
virtually nothing at the shoreline

A good video of infragravity waves hitting Cornwall, England, on 10 March 2008, can be seenHERE